US 2014/0340190 A1 discloses a wireless control system, wherein different wall-switches control different ceiling-lamps. Each ceiling-lamp has a timer for measuring an amount of elapsed time from a moment of switching on the ceiling-lamp via its wall-switch. One of the ceiling-lamps can become a master, and the other ones then become slaves that upon reception of identifications from the master and therefore upon request from the master send their amounts of elapsed time to the master. By comparing the amounts of elapsed time of the slaves, the master can determine which slaves belong to the same wall-switch as the master.